Positive displacement pumps are characterized by alternately filling and emptying an enclosed volume by the operation of a mechanism such as a reciprocating piston, meshing gears, sliding vanes, screws, etc. The pumping mechanism, e.g., a reciprocating piston, is movable within an enclosed chamber between an intake position in which negative pressure is created within the chamber to draw fluid therein, and an exhaust position in which the fluid drawn into the chamber is pressurized and/or exhausted through an outlet in the chamber. In many instances, the pumping mechanism is driven by an electric motor through a crank, or, alternatively, the driving force for the pumping mechanism can be direct acting such as by steam or compressed air.
Positive displacement pumps of the type described above have a number of limitations, particularly for certain types of applications. One problem is that the operation of the pumping mechanisms is relatively loud. Reciprocating pistons or plungers, even if well lubricated, are relatively noisy when sliding within an enclosed chamber. Similarly, the pumping mechanisms associated with rotary-type displacement pumps, e.g., meshing gears, sliding vanes or screws, are also relatively noisy due to the metal-to-metal engagement of their moving parts.
A second problem with the positive displacement pumps such as described above is that the pumping mechanisms and associated bearings must be lubricated to reduce wear and ensure smooth operation of the pump. As a result, the fluid being pumped comes into contact with the lubricated surfaces of the pumping mechanisms and can pick up contaminants. This is unacceptable where the air or other fluid being pumped must be clean such as in the pumping of oxygen into oxygen tents and similar applications in hospitals or other health care facilities. It is also important for pumps utilized in hospitals to operate quietly, and vibration-free, which is another deficiency of prior art positive displacement pumps.
A third problem with prior art positive displacement pumps is their limited capability to dissipate heat generated by the moving parts. Particularly at high operating speeds, the compression of the air being pumped, and the metal-to-metal contact between the pumping mechanisms, e.g., reciprocating pistons, meshing gears, etc., generates heat which is relatively slowly dissipated from such working parts through the walls of the enclosed chamber. After a period of operation, the temperature of the interior of the chamber may increase substantially leading to damage of the pump.
Another problem with prior art positive displacement pumps involves restarting the pump after it has been operated for a period of time and then shut down. Under these circumstances, the lines between the pump and fluid supply remain pressurized and make it difficult to initially move the pumping mechanism, e.g., a reciprocating piston, to overcome such "dead-head" or back pressure. This problem has been solved in the prior art by incorporating a bleeder valve or other pressure relief device between the pump and fluid supply to eliminate back pressure, but such devices add to the cost and complexity of the pumping system.
Another type of positive displacement pump has been proposed in the prior art which is shown, for example, in U.S. Pat. Nos. 901,344 to Horstmann; 1,511,985 to Spencer; 3,465,684 to Moll; and, 4,169,433 to Crocker. These pumps employ a "floating" piston which is freely movable within a cylinder having an inlet and an outlet. Movement of the piston within the cylinder in one direction creates a negative pressure therein which draws air through the inlet into the cylinder. Movement of the piston in the opposite direction forces the fluid through the outlet of the cylinder to create a pumping action.
Movement of the floating piston within the cylinder is caused by rotating the cylinder and piston about a first axis so that centrifugal force is applied to the piston, and at the same time rotating the cylinder about an axis passing through its midpoint so that the ends of the cylinder change position relative to the first axis. With the ends of the cylinder in one position, the piston is thrown radially outwardly toward one end of the cylinder by centrifugal force, and this movement either intakes or exhausts fluid from the cylinder. The cylinder is then rotated about its midpoint so that its ends switch position, which, in turn, causes the piston to be moved by centrifugal force to the opposite end of the cylinder.
One problem with centrifugal force, positive displacement pumps of the type described above is that a high amount of energy is required to rotate the cylinder about its midpoint in order to move the piston from one end of the cylinder to the other. Each time the piston changes position within the cylinder, the center of gravity of the cylinder and piston unit changes. Substantial power is required to change this center of gravity, i.e., to overcome centrifugal force and cause the piston to shift from one end of the cylinder to the other. This problem is made even worse when the cylinder and piston are rotated at high speeds in order to increase the pumping rate of the pump. The higher the speed of rotation the higher the centrifugal force applied to the piston, which, in turn, requires more energy to rotate the cylinder about its midpoint and shift the position of the piston therewithin.
Centrifugal-type, positive displacement pumps also share many of the deficiencies of standard positive displacement pumps. They are relatively noisy where the floating piston is allowed to contact the ends of the cylinder, and the fluid being pumped is exposed to and can be contaminated by the lubricant which permits movement of the piston within the cylinder.